Method of manufacturing a flexible printed circuit board

ABSTRACT

An FPCB and a method of manufacturing the same, in which an electrical signal-conductive portion of the FPCB is subjected to little stress so as not to be broken by fatigue in spite of repeated bending of the FPCB, thereby increasing the lifetime of the FPCB.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. divisional application filed under 37 CFR1.53(b) claiming priority benefit of U.S. Ser. No. 12/213,570, filed inthe United States on Jun. 20, 2008, now allowed, which claims earlierpriority benefit to Korean Patent Application No. 10-2007-133299 filedwith the Korean Intellectual Property Office on Dec. 18, 2007, thedisclosures of which are incorporated herein by reference.

BACKGROUND

1. Field

The present invention relates to a flexible printed circuit board (FPCB)and a manufacturing method thereof, and more particularly, to an FPCBused in various electrical and/or electronic devices such as a mobilecommunication device and a method of manufacturing the same.

2. Description of the Related Art

In general, an FPCB is a circuit board that is designed to conductvarious complicated signals, and includes a conductor layer forconducting electrical signals and dielectric layers covering the topsurface and the underside surface of the conductive layer. This type ofFPCB can be manufactured to be thin and small, and is used for a thinand small electrical device such as a mobile communication device.

The FPCB has highly-flexible physical property, and thus is used for ashape-changeable electrical device. Particularly, the FPCB is used forelectrical circuit connection between a body and a slide (or a foldablemember) in a slide-type or folder-type mobile communication device.

That is, when the slide (or the foldable member) is moved open orclosed, the FPCB flexibly bends, so that the circuit can be continuouslyconnected.

As mentioned above, the FPCB maintains electrical circuit connectionwhen it repeats bending since it is used in a shape-changeable devicesuch as a mobile communication device.

However, when the FPCB is repeatedly bending, a conductor layer insidethe FPCB suffers from fatigue, caused by the repeated bending. This as aresult causes cracks in the conductor layer, thereby reducing thelifetime of the FPCB.

SUMMARY

The present invention has been made to solve the foregoing problems withthe prior art, and therefore the present invention is directed to anFPCB and a method of manufacturing the same, in which an electricalsignal-conductive portion of the FPCB is subjected to little stress soas not to be broken by fatigue in spite of repeated bending of the FPCB,thereby increasing the lifetime of the FPCB.

According to an aspect of the present invention, the FPCB includes afirst dielectric portion forming a dielectric layer; a second dielectricportion forming a dielectric layer opposite the first dielectricportion; and a conductive portion conducting electrical signals andhaving a neutral plane, wherein the neutral plane is located within apredetermined range of the thickness of the conductive portion, and hassubstantially zero strain due to bending between the first and seconddielectric portions.

On both sides of the neutral plane, the first dielectric portion issubstantially subjected to tension and the second dielectric portion issubstantially subjected to compression.

According to another aspect of the present invention, the FPCB includesa first dielectric portion subjected to tension in response to bending;a second dielectric portion subjected to compression in response tobending; and a conductive portion conducting electrical signals, andhaving a neutral plane, the neutral plane is within a predeterminedrange of the thickness of the conductive portion, and forms asubstantial interface of tension and compression between the first andsecond dielectric portions.

Each of the first and second dielectric portions may include at leastone dielectric layer.

Here, a geometric center, obtained by converting the thickness and thewidth of respective layer of the flexible printed circuit board usingsame elastic modulus, may be located in the predetermined range of thethickness of the conductive portion.

The position of the neutral plane may be determined according to thefollowing Formula:

Y _(n)=(Σ(T _(i) *E _(i) *Y _(i)))/(Σ(T _(i)*)E _(i))), and

ΣT _(x) <Y _(n)<(ΣT _(x) +T _(c))

where Y_(n) is a distance normally extending from a reference plane tothe neutral plane, T_(i) is the thickness of a respective layer of theflexible printed circuit board, E_(i) is an elastic modulus of amaterial placed in the respective layer of the flexible printed circuitboard, Yi is a distance normally extending from the reference plane tothe geometric center of the respective layer, T_(x) is the thickness ofthe respective layer under the conductive portion, T_(c) is thethickness of the conductive portion, and the reference plane is anunderside of the flexible printed circuit board.

The neutral plane may be located in a range determined by the followingFormula:

(ΣT _(x)+0.2T _(c))≦Y _(n)≦(ΣT _(x)+0.8T _(c)).

According to a further aspect of the present invention, the FPCBincludes a conductive member including a conductive portion, whichconducts electrical signals, and at least one dielectric layer; and acover including a adhesive layer adhered to the conductive portion andat least one dielectric layer, wherein the thickness and the elasticmodulus of the conductive member and the cover are determined so that aneutral plane is placed within a predetermined range of the thickness ofconductive portion, the neutral plane being substantially free fromtension or compression in response to bending of the conductive memberand the cover.

The position of the neutral plane may be determined according to thefollowing Formula:

Y _(n)=(Σ(T _(i) *E _(i) *Y _(i)))/(Σ(T _(i)*)E _(i))), and

ΣT _(x) <Y _(n)<(ΣT _(x) +T _(c)),

where Y_(n) is a distance normally extending from a reference plane tothe neutral plane, T_(i) is the thickness of a respective layer of theflexible printed circuit board, E_(i) is an elastic modulus of amaterial placed in the respective layer of the flexible printed circuitboard, Y_(i) is a distance normally extending from the reference planeto the geometric center of the respective layer, T_(x) is the thicknessof the respective layer under the conductive portion, T_(c) is thethickness of the conductive portion, and the reference plane is anunderside of the flexible printed circuit board.

The neutral plane may be located in a range determined by the followingFormula:

(ΣT _(x)+0.2T _(c))≦Y _(n)≦(ΣT _(x)+0.8T _(c)).

According to another aspect of the present invention, the method ofmanufacturing an FPCB includes determining the elastic modulus of aconductive portion and the elastic modulus of first and seconddielectric portions, which insulate the conductive portion; determiningthe thickness of the conductive portion and the first and seconddielectric portions so that a neutral plane is located within apredetermined range of the thickness of the conductive portion, theneutral plane being substantially free from tension or compression inresponse to bending of the conductive member and the cover; andinsulating the conductive portion according to the determined thicknessand the determined elastic modulus.

The step of determining the thickness may design to locate a geometriccenter in the predetermined range of the thickness of the conductiveportion, the geometric center obtained by converting the thickness andthe width of respective layer of the first dielectric portion, thesecond dielectric portion and the conductive portion using same elasticmodulus.

The thickness of the first dielectric portion, the second dielectricportion and the conductive portion may be determined according to theposition of the neutral position, which is obtained by the followingFormula:

Y _(n)=(Σ(T _(i) *E _(i) *Y _(i)))/(Σ(T _(i)*)E _(i))), and

ΣT _(x) <Y _(n)<(ΣT _(x) +T _(c)),

where Yn is a distance normally extending from a reference plane to theneutral plane, Ti is the thickness of a respective layer of the flexibleprinted circuit board, Ei is an elastic modulus of a material placed inthe respective layer of the flexible printed circuit board, Yi is adistance normally extending from the reference plane to the geometriccenter of the respective layer, Tx is the thickness of the respectivelayer under the conductive portion, Tc is the thickness of theconductive portion, and the reference plane is an underside of theflexible printed circuit board.

The step of determining the thickness may locate the neutral plane in arange determined by the following Formula:

(ΣT _(x)+0.2T _(c))≦Y _(n)≦(ΣT _(x)+0.8T _(c))

According to yet another aspect of the present invention, the method ofmanufacturing an FPCB includes adhering a cover, which includes aadhesive layer and at least one dielectric layer, to a conductivemember, which includes a conductive portion and at least one dielectriclayer; determining the elastic modulus and the thickness of a shield tobe layered on at least one of the conductive member and the cover, sothat a neutral plane is located within a predetermined range of thethickness of the conductive portion, he neutral plane forming asubstantial interface of tension and compression in response to bending;and layering the shield on at least one of the conductive member and thecover, with the elastic modulus and the thickness of the shielddetermined so that the neutral plane is located in the conductiveportion.

The shield may include a first shield layered on a top surface of thecover; and a second shield layered on an underside of the conductivemember.

Here, the elastic modulus and the thickness of the shield are determinedaccording to the following Formula:

Y _(n)=(Σ(T _(i) *E _(i) *Y _(i)))/(Σ(T _(i)*)E _(i))), and

ΣT _(x) <Y _(n)<(ΣT _(x) +T _(c))

where Y_(n) is a distance normally extending from a reference plane tothe neutral plane, T_(i) is the thickness of a respective layer of theflexible printed circuit board, E_(i) is an elastic modulus of amaterial placed in the respective layer of the flexible printed circuitboard, Y_(i) is a distance normally extending from the reference planeto the geometric center of the respective layer, T_(x) is the thicknessof the respective layer under the conductive portion, T_(c) is thethickness of the conductive portion, and the reference plane is anunderside of the flexible printed circuit board.

The step of determining the thickness and elastic modulus of the shieldmay locate the neutral plane in a range determined by the followingFormula:

(ΣT _(x)+0.2T _(c))≦Y _(n)≦(ΣT _(x)+0.8T _(c))

In the FPCB of the present invention, even though the FPCB is repeatedlybent, the conductive portion, which conducts electrical signals, is noteasily fractured by fatigue caused by stress, thereby increasing thelifetime of the FPCB. The manufacturing method of the present inventioncan newly manufacture an FPCB having the above-described properties, andalso provide the above-describe properties to a previously-made FPCB.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIGS. 1 and 2 are perspective views illustrating example operations ofan FPCB according to an embodiment of the present invention;

FIG. 3 is a cross-sectional view of the FPCB, taken along the line I-Iin FIG. 2;

FIG. 4 is a configuration view illustrating stress distribution when anobject is bent or curved;

FIG. 5 is a cross-sectional view of an FPCB according to an embodimentof the present invention, illustrating physical properties of materialsof respective layers of the FPCB;

FIG. 6 is a graph illustrating the effects of an FPCB according to anembodiment of the present invention; and

FIGS. 7 and 8 are cross-sectional views illustrating a method ofmanufacturing an FPCB according to another embodiment of the presentinvention.

DESCRIPTION OF EMBODIMENTS

Exemplary embodiments of the present invention will now be described indetail with reference to the accompanying drawings.

FIGS. 1 and 2 illustrate the operation of a flexible printed circuitboard (FPCB) according to an embodiment of the present invention. First,the FPCB according to an embodiment of the present invention will beschematically described with reference to FIGS. 1 and 2.

The FPCB illustrated in FIGS. 1 and 2 comprises first and secondconnectors 1 and 2, each of which is connected to a main circuit boardor an auxiliary circuit board, and a flexible portion 3, which transmitselectrical signals between the connectors 1 and 2 and is bent flexibly.

The FPCB illustrated in FIGS. 1 and 2 is characterized in that it canrealize various electronics, particularly compact slim devices. Forexample, the FPCB can be applied to a slide type mobile communicationterminal.

In other words, assuming that the first connector 1 is connected to aPCB installed in a terminal body (not shown) and that the secondconnector 2 is connected to a slide (not shown) performing operationssuch as slide-up/down on the terminal body, the state illustrated inFIG. 1 can be regarded to be a slide-up state, whereas the stateillustrated in FIG. 2 can be regarded to be a slide-down state.

A user repeats the slide-up and the slide-down while using the slidetype mobile communication terminal. In this case, the flexible portion 3repeats the flexure as illustrated in FIGS. 1 and 2.

In this manner, in the case in which the FPCB according to an embodimentof the present invention is bent, the flexible portion 3 is subjected totension and compression at the same time. A detailed mechanism of theoperation will be described with reference to FIG. 3.

FIG. 3 is a cross-sectional view taken the line I-I of the FPCBillustrated in FIG. 1 or 2.

The FPCB is fundamentally divided into a conductive portion fortransmitting electrical signals, and a dielectric portion for insulatingthe conductive portion. As illustrated in FIG. 3, the FPCB according toan embodiment of the present invention comprises a first dielectricportion 10, a conductive portion 30, and a second dielectric portion 20.

The first and second dielectric portions 10 and 20 are disposed with theconductive portion 30 in between, and serve to protect and insulate theconductive portion 30.

When the FPCB is bent as illustrated in FIGS. 1 and 2, tension T andcompression C thereof occurs at the same time as illustrated in FIG. 3.

In other words, the tension T occurs at the first dielectric portion 10,and the compression C occurs at the second dielectric portion 20. In theembodiment illustrated in FIG. 3, the first dielectric portion 10includes a first layer 11, a second layer 12, and a third layer 13, andthe second dielectric portion 20 includes a fourth layer 24 and a fifthlayer 25.

The embodiment of FIG. 3 illustrates that each of the first and seconddielectric portions 10 and 20 has, but not limited to, a plurality ofdielectric layers. For example, each of the first and second dielectricportions 10 and 20 may include a single dielectric layer.

More specifically, in the embodiment illustrated in FIG. 3, the firstlayer 11 of the first dielectric portion 10 is an electromagneticinterference (EMI) shield layer, the second layer 12 is a polyimidelayer, and the third layer 13 is an adhesive layer for adhering to theconductive portion 30.

Further, the fourth layer 24 of the second dielectric portion 20 is apolyimide layer, and the fifth layer 25 is an EMI shield layer.

However, the FPCB according to an embodiment of the present invention isnot essentially divided into the first dielectric portion 10, the seconddielectric portion 20, and the conductive portion 30. Thus, the FPCBaccording to another embodiment of the present invention can be dividedinto a conductive member 50 and a cover 40.

The conductive member 50 includes the conductive portion 30 in additionto the fourth and fifth layers 24 and 25, both of which are stackedbeneath the conductive portion 30. The cover 40 includes the first,second and third layers 11, 12 and 13, which are stacked in that order.At this time, the third layer 13 typically serves as an adhesive layerfor adhering to the conductive portion 30.

Meanwhile, the embodiment illustrated in FIG. 3 includes a neutral plane35, which is substantially subjected neither to the tension nor thecompression.

Any material will be extended or contracted to a certain extent undertensile or compressive stress. However, the neutral plane 35substantially causes neither tension nor compression, so that the strainthereof substantially becomes zero.

Here, the strain is expressed by a ratio of an original length or volumeto a deformed length or volume. Since no change is caused from theneutral plane 35, the strain of the neutral plane 35 substantiallybecomes zero.

In other words, as illustrated in FIG. 3, when the FPCB is bent, thefirst dielectric portion 10 (or the cover 40) is subjected to thetension T, and the second dielectric portion 20 (or the conductivemember 50, from which the conductive portion 30 is excluded) issubjected to the compression C. Further, the neutral plane 35substantially causes neither the tension nor the compression.

Thus, according to an embodiment of the present invention, since theneutral plane 35 is disposed within the conductive portion 30, the FPCBexperiences the tension and the compression to the minimum extent inspite of the repeated bending thereof, very slight deformation, and thusweak fatigue. Accordingly, the FPCB can inhibit cracks caused by thefatigue, and thus increase its life span.

In detail, the neutral plane 35 is preferably adapted to exist within apredetermined range of the thickness of the conductive portion 30. Tothis end, each of the first dielectric portion 10, the second dielectricportion 20 and the conductive portion 30 must be determined with respectto a thickness, an elastic modulus, etc. thereof.

A method of determining the thicknesses and the elastic moduli of therespective layers constituting the FPCB will be described with referenceto FIGS. 4 and 5.

FIG. 4 illustrates stress distribution when an object is bent or curved,and FIG. 5 illustrates physical properties of materials of therespective layers of an FPCB according to an embodiment of the presentinvention.

As illustrated in FIG. 4, it is assumed that X and Y materials arestacked, that width and elastic modulus of the X material are b and Exrespectively, and that width and elastic modulus of the Y material are band E_(y) respectively. When the bending occurs from top to bottom, theneutral plane N is located at a substantial interface between thetension T and the compression C.

Further, the neutral plane N is not matched with a boundary between theX and Y materials. This is because area and elastic modulus of the Xmaterial are different from those of the Y material.

Thus, as illustrated in FIG. 4, the position of the neutral plane can beeasily obtained by calculation based on the elastic modulus of onematerial.

In other words, when calculated on the basis of the elastic modulus Exof the X material, the width of the Y material is nb, wheren=E_(y)/E_(x).

The neutral plane N is a line passing through the geometric centers ofthe X and Y materials which are represented by the width calculated inthis way.

However, since the FPCB has various layers such as the conductiveportion, the dielectric layer for insulating the conductive portion, theshield layers for shielding the EMI, and so on, the elastic moduli andthicknesses of which are different from each other, it is difficult tofind the whole geometric centers.

Here, in order to find the position of the neutral plane, i.e. the linepassing through the geometric centers of the materials represented bycalculation based on the same elastic modulus, the concept of thegeometrical moment of area (i.e. the moment of area) is used. Thisconcept will be described below in greater detail.

The FPCB of the present invention is designed so that the neutral planeis located in the conductive portion as described above, namely within apredetermined range of the thickness of the conductive portion.

The positional relationship between the neutral plane and the conductiveportion of the FPCB according to an embodiment of the present inventionwill be described in greater detail with reference to FIG. 5. Theembodiment illustrated in FIG. 5 is merely provided for the illustrativepurpose. Thus, the present invention is not limited to this embodiment,and includes the case where each of the first and second dielectricportions has one dielectric layer as well as two or more dielectriclayers.

As illustrated in FIG. 5, the thickness and the elastic modulus of thefirst layer 11 of the FPCB according to an embodiment of the presentinvention are T₁ and E₁, respectively.

The thickness and the elastic modulus of the second layer 12 are T₂ andE₂, respectively. The thickness and the elastic modulus of the thirdlayer 13 are T₃ and E₃, respectively. Further, the thickness and theelastic modulus of the conductive portion 30 are T_(c) and E_(c),respectively.

The thickness and the elastic modulus of the fourth layer 24 are T₄ andE₄, respectively. The thickness and the elastic modulus of the fifthlayer 25 are T₅ and E₅, respectively.

The widths of these components are equal to each other, and areexpressed by L. Meanwhile, assuming that one outer surface, for instancea lower surface, of the FPCB illustrated in FIG. 5 is a referencesurface, the distances from the reference surface to the geometriccenters of the respective layers can be expressed by Y_(i), where “i” isthe symbol denoting the respective layers. Specifically, the symbol “i”denotes one of 1, 2, 3, c, 4 and 5.

Here, the geometric centers of the respective layers, of which the FPCBis composed, refer to the distances from the reference surface to thecentral lines of the respective layers (i.e. lines passing through thecenters of thickness of the respective layers) in an uncalculated state,rather than the geometric centers of the respective layers calculated onthe basis of the same elastic modulus.

In detail, the distance from the reference surface to the geometriccenter of the first layer 11 is Y₁, the distance from the referencesurface to the geometric center of the second layer 12 is Y₂, thedistance from the reference surface to the geometric center of the thirdlayer 13 is Y₃, the distance from the reference surface to the geometriccenter of the fourth layer 24 is Y₄, and the distance from the referencesurface to the geometric center of the fifth layer 25 is Y₅. Further,the distance from the reference surface to the neutral plane 35 isexpressed by Y_(n).

Thus, Y₁ is T₁/2+T₂+T₃+T_(C)+T₄+T₅, and Y₂ is T₂/2+T₃+T_(C)+T₄+T₅.According to the embodiment illustrated in FIG. 5, Yi, i.e. one of thevalues ranging from Y1 to Y5, is as shown in Tables 1 and 2.

In order to show the positional relationship between the neutral planeand the conductive portion of the FPCB illustrated in FIG. 5, it isnecessary to calculate the geometrical moments of area (i.e. the momentsof area) of the respective layers of which the FPCB is composed. Here,the geometrical moment of area is defined as the product of the area bythe geometric center.

The geometrical moment of area is the concept that is introduced inorder to obtain the geometric centers of the whole materials representedby calculating the respective layers, of which the FPCB according to thepresent invention is composed, on the basis of the same elastic modulus(At this time, the materials represented by calculating the respectivelayers on the basis of the same elastic modulus may be further increasedor decreased in area).

For example, when conversion factors represented by calculating theelastic moduli of the second layer 12, the third layer 13, theconductive portion 30, the fourth layer 24, and the fifth layer 25 onthe basis of the elastic modulus of the first layer 11 are applied tothe respective layers in FIG. 5, the widths of the respective layers arechanged after this calculation. Thus, when the calculated widths areapplied to the respective layers, some of the layers are calculated thatthe areas thereof are further increased, and the other layers arecalculated that the areas thereof are further decreased.

When the geometric centers of the materials are found in overallconsideration of the calculated materials, the geometric centers arematched with the neutral plane.

In order to obtain the geometric center for the whole materials, theconcept of the geometrical moment of area is introduced. Thus, thegeometric center is the ratio of an entire area to an entire geometricalmoment of area.

The entire area is the cross-sectional area of the materials of therespective calculated layers, or the sum of cross-sectional areas of therespective calculated layers. The entire geometrical moment of area isthe sum of the geometrical moments of area of the respective calculatedlayers.

The following Table 1 shows thicknesses, elastic moduli, conversionfactors n, conversion widths, conversion areas, and geometric centers ofthe respective layers of the FPCB illustrated in FIG. 5. Here, theconversion is based on the elastic modulus E₁ of the material of whichthe first layer 11 is formed.

Further, the geometric centers refer to the distances from the referencesurface to the central lines of the respective layers in an uncalculatedstate.

TABLE 1 T* EM* CF* CW* CA* Geometric center 1^(st) Layer T₁ E₁ 1 L T₁ *L (T₁/2 + T₂ + T₃ + T_(c) + (EMI) T₄ + T₅) 2^(nd) Layer T₂ E₂ E₂/E₁ L *(E₂/E₁) T₂ * L * (E₂/E₁) (T₂/2 + T₃ + T_(c) + T₄ + T₅) (polyimide)3^(rd) Layer T₃ E₃ E₃/E₁ L * (E₃/E₁) T₃ * L * (E₃/E₁) (T₃/2 + T_(c)+T₄ + T₅) (adhesive layer) Conductive T_(c) E_(c) E_(c)/E₁ L * (E_(c)/E₁)T_(c) * L * (E_(c)/E₁) (T_(c)/2 + T₄ + T₅) portion (Cu) 4^(th) Layer T₄E₄ E₄/E₁ L * (E₄/E₁) T₄ * L * (E₄/E₁) (T₄/2 + T₅) (polyimide) 5^(th)Layer T₅ E₅ E₅/E₁ L * (E₅/E₁) T₅ * L * (E₅/E₁) (T₅/2) (EMI) Note) T * :thickness, EM * : elastic modulus, CF * : conversion factor (n), CW * :conversion width, CA * : conversion area

With regard to the calculation of the respective layers, in the presentembodiment, the conversion factor n is applied to the width L, therebyexpressing the conversion width. Alternatively, the conversion factormay be applied to the thickness instead of the width, the result ofwhich is the same.

Meanwhile, the following Table 2 shows conversion areas, geometriccenters, and the geometrical moments of area. Here, A_(i) and M_(i)denote the conversion area of each layer and the geometrical moment ofarea of each layer respectively, where “i′ is the symbol denoting eachlayer.

TABLE 2 CA* Geometric center GMA* 1^(st) Layer A₁ = T₁*L Y₁ = (T₁/2 +T₂ + T₃ + T_(C) + M₁ = A₁*Y₁ = T₁*L*(T₁/2 + (EMI) T₄ + T₅) T₂ + T₃ +T_(C) + T₄ + T₅) 2^(nd) Layer A₂ = T₂*L*(E₂/E₁) Y₂ = (T₂/2 + T₃ +T_(C) + T₄ + M₂ = A₂*Y₂ = T₂*L*(E₂/E₁)* (polyimide) T₅) (T₂/2 + T₃ +T_(C) + T₄ + T₅) 3^(rd) Layer* A₃ = T₃*L*(E₃/E₁) Y₃ = (T₃/2 + T_(C) +T₄ + T₅) M₃ = A₃*Y₃ = T₃*L*(E₃/E₁)* (adhesive layer) (T₃/2 + T_(C) +T₄ + T₅) Conductive A_(c) = T_(c)*L*(E_(c)/E₁) Y_(c) = (T_(c)/2 + T₄ +T₅) M_(c) = A_(c)*Y_(c) = T_(c)*L*(E_(c)/E₁)* portion (Cu) (T_(c)/2 +T₄ + T₅) 4^(th) Layer A₄ = T₄*L*(E₄/E₁) Y₄ = (T₄/2 + T₅) M₄ = A₄*Y₄ =T₄*L*(E₄/E₁)* (polyimide) (T₂/2 + T₅) 5^(th) Layer A₅ = T₅*L*(E₅/E₁) Y₅= (T₅/2) M₅ = A₅*Y₅ = T₅*L*(E₅/E₁)* (EMI) (T₅/2) Note) CA*: conversionarea, GMA*: Geometrical Moment of Area = Area*Geometric center

As shown in Tables 1 and 2, the total area and the total geometricalmoment of area for all the layers of the FPCB illustrated in FIG. 5 areexpressed as follows:

A=ΣA_(i), and M=ΣM_(i)

The position of the neutral plane 35 can be expressed as follows.

Yn=M/A=(ΣM _(i))/(ΣA _(i))

When these formulas are applied in detail, the total area A can beexpressed as follows:

$\begin{matrix}{{\Sigma \; A_{i}} = \left( {A_{1} + A_{2} + A_{3} + A_{c} + A_{4} + A_{5}} \right)} \\{= {L*{\begin{pmatrix}{{T_{1}*E_{1}} + {T_{2}*E_{2}} + {T_{3}*E_{3}} +} \\{{T_{c}*E_{c}} + {T_{4\;}*E_{4}} + {T_{5}*E_{5}}}\end{pmatrix}/E_{1}}}}\end{matrix}$

The total moment M can be expressed as follows:

$\begin{matrix}{{\Sigma \; M_{i}} = \left( {M_{1} + M_{2} + M_{3} + M_{c} + M_{4} + M_{5}} \right)} \\{= \left( {{A_{1}*Y_{1}} + {A_{2}*Y_{2}} + {A_{3}*Y_{3}} + A_{c} + {A_{4}*Y_{4}} + {A_{5}*Y_{5}}} \right)} \\{= {L*{\begin{pmatrix}\begin{matrix}{{T_{1}*E_{1}*Y_{1}} + {T_{2}*E_{2}*Y_{2}} +} \\{{T_{3}*E_{3}*Y_{3}} + {T_{c}*E_{c}*Y_{c}} +}\end{matrix} \\{{T_{4}*E_{4}*Y_{4}} + {T_{5}*E_{5}*Y_{5}}}\end{pmatrix}/E_{1}}}}\end{matrix}$

The position Y_(n) of the neutral plane 35 can be expressed by thefollowing formula (hereinafter, referred to as “Formula 1”).

$\begin{matrix}{{Yn} = {\left( {\Sigma \; M_{i}} \right)/\left( {\Sigma \; A_{i}} \right)}} \\{= {{T_{1}*E_{1}*Y_{1}} + {T_{2}*E_{2}*Y_{2}} + {T_{3}*E_{3}*Y_{3}} + {T_{c}*E_{c}*Y_{c}} +}} \\{\left. {{T_{4}*E_{4}*Y_{4}} + {T_{5}*E_{5}*Y_{5}}} \right) \div} \\{\left( {{T_{1}*E_{1}} + {T_{2}*E_{2}} + {T_{3}*E_{3}} + {E_{c}*E_{c}}\; + {T_{4}*E_{4}} + {T_{5}*E_{5}}} \right)} \\{= {\left( {\Sigma \left( {T_{i\;}*E_{i}*Y_{i\;}} \right)} \right) \div \left( {\Sigma \left( {T_{i}*E_{i}} \right)} \right)}}\end{matrix}$

The neutral plane 35 defined by Formula 1 is designed to be within thethickness range of the conductive portion 30, and preferably to satisfythe following formula (hereinafter, referred to as “Formula 2”).

ΣT _(x) <Y _(n)<(ΣT _(x) +T _(c))   Formula 2

Here, T_(x) indicates the thickness of each dielectric layer under theconductive portion 30. In the embodiment illustrated in FIG. 5, T_(x) isthe sum of T₄ and T₅.

In other words, T_(i), E_(i), and Y_(i) are determined such that Y_(n)is greater than T₄+T₅ and is smaller than T₄+T₅+T_(c). Thus, Yn existsbetween the lower and upper surfaces of the conductive portion 30.

For example, if the materials of the first dielectric portion 10, thesecond dielectric portion 20, and the conductive portion 30 arepredetermined before the FPCB is manufactured, the elastic modulus E_(i)of each layer constituting each component is a fixed value, and thethickness of each layer constituting each of the first dielectricportion 10, the second dielectric portion 20, and the conductive portionis determined by Formulas 1 and 2.

The determination of the thickness is conducted by a trial and errormethod, or by changing the thickness of a changeable layer on the basisof the aforementioned formulas if the thicknesses of some of the layersare fixed, i.e., are not changed. Thereby, the FPCB according to anembodiment of the present invention can be manufactured.

Further, in the case of the cover 40 and the conductive member 50, theelastic modulus is determined by the method similar to theaforementioned method, and the thickness of each layer is determined.The cover 40 and the conductive member 50, prepared on the basis of thisdetermination, are adhered to each other. Thereby, the FPCB according tothe present invention can be manufactured.

However, the FPCB according to the present invention has a very thinstructure in which its thickness is several microns. As such, althoughthe FPCB is manufactured on the basis of the thicknesses determined bythe aforementioned formulas, there is no alternative but to allow atolerance to some extent.

Thus, the position Y_(n) of the neutral plane 35 is preferably adaptedto be within a range limited to some extent in spite of the thicknessrange of the conductive portion 30. Thereby, the neutral plane 35 can bematched with the geometric center of the conductive portion 30 as exactas possible, or be spaced apart from the geometric center of theconductive portion 30 as near as possible.

Thus, as illustrated in FIG. 5, the position Yn of the neutral plane ispreferably adapted to be determined by the following formula.

D₁≦Y_(n)≦D₂

Here, D1 is preferably selected by a distance ranging from the referencesurface to a position corresponding to about 20% of the thickness of theconductive portion, and D2 is preferably selected by a distance rangingfrom the reference surface to a position corresponding to about 80% ofthe thickness of the conductive portion. In other words, the positionY_(n) of the neutral plane is preferably adapted to be determined withina range expressed by the following formula (hereinafter, referred to as“Formula 3”).

(ΣT _(x)+0.2T _(c))≦Y _(n)≦(ΣT _(x)+0.8T _(c))

The reason D1 and D2 are determined as described above is for selectinga more stable range in consideration of the tolerance caused by themanufacturing, rather than the present invention has no effect beyondthe range.

Meanwhile, the effects of the FPCB according to an embodiment of thepresent invention will be described with reference to FIG. 6. FIG. 6shows the results of lifespan prediction simulation performed on an FPCBaccording to an embodiment of the present invention and a known FPCB.

In the graph shown in FIG. 6, the abscissa denotes a distance betweenthe geometric center of the conductive portion and the neutral plane inunit of micron, and the ordinate denotes the number of times ofrepeating bending using a slide.

Further, the symbol A corresponds to the case where the geometric centerof the conductive portion and the neutral plane of the FPCB according toan embodiment of the present invention are matched with each other, andthe symbol B denotes the result of a bending test of the known FPCB.

The graph shown in FIG. 6 is obtained by simulating the case where thethickness T_(c) of the conductive portion 30 is set to 12 μm. When thevalue of the abscissa is zero (0), this denotes the case where theneutral plane is matched with the geometric center of the conductiveportion. As it proceeds to the right side on the basis of the zero valueof the abscissa, this denotes that the neutral plane is gradually spacedapart from the geometric center of the conductive portion in an upwardor downward direction.

For example, if the neutral plane is spaced 5 μm apart from thegeometric center of the conductive portion in a downward direction, thenumber of times of repeating the bending using the slide correspondingto 5 of the abscissa on the graph shown in FIG. 6 becomes a predictedlifespan. If the neutral plane is spaced 5 μm apart from the geometriccenter of the conductive portion in an upward direction, the number oftimes of repeating the bending using the slide corresponding to 5 on theabscissa of the graph shown in FIG. 6 also becomes a predicted lifespan.

In other words, the range from 0 μm to 6 μm on the abscissa of the graphshown in FIG. 6 is the thickness range of the conductive portion. Thesymbol P of the graph denotes the range between D₁ and D₂ illustrated inFIG. 5.

As shown in FIG. 6, although the neutral plane is within the thicknessrange (from 0 μm to 6 μm) of the conductive portion, the lifetime of theFPCB of the present invention is longer than that of the conventionalFPCB. Even in the range of P, it is possible to achieve better effectsthan the conventional FPCB.

When the range of D₁ and D₂ are selected to be between 20% and 80% ofthe thickness of the conductive portion as described above, P in thegraph of FIG. 6 approximately ranges from 0 to 4.8.

The present invention aims to locate the conductive portion in a plane,the elastic modulus of which is substantially zero (0). This planeindicates the interface between one portion under tension and anotherportion under compression when bending occurs, or the neutral planewhich is not subjected to tension or compression in response to bending.Here, the neutral plane has zero (0) elastic modulus.

Since the conductive portion has a predetermined thickness, all of theconductive portion cannot serve as the neutral plane. When the FPCB ofthe present invention is bent, a small amount of tension and compressionwill be applied to the conductive portion, thereby deforming the same ina very small range.

The present invention is directed to finally match the neutral plane tothe geometric center of the conductive portion. When the neutral planeis not consistent with but close vicinity to the geometric center of theconductive portion, the conductive portion is not free from stress orstrain in the bending. However, as shown in FIG. 6, the stress appliedto the conductive portion will be very small, and the strain will beclose to 0, so that substantially no deformation occurs in theconductive portion, thereby increasing the lifetime of the FPCB.

Table 3 below shows the result of lifetime experiments, performed onFPCB specimens according to an embodiment of the present invention.Here, the distance between the geometric center and the neutral planewas 4.00 μm in four of the specimens, and 1.26 μm in two of thespecimens.

TABLE 3 Distance from neutral plane 1 2 3 4 4.00 μm 163,600 157,400173,300 151,100 1.26 μm 331,800 354,600

From the data reported in Table 3 above, it can be appreciated that theaverage lifetime of the 4.00 μm specimens is 161,350 times, and theaverage lifetime of the 1.26 μm specimens is 343,200 times.

That is, the experiments reveal that the lifetime of the conductiveportion against bending may increase as the geometric center thereof isfarther from the neutral plane.

Referring to the drawings, the method of manufacturing an FPCB of thepresent invention will be described in more detail.

The method of manufacturing an FPCB of the present invention basicallyincludes determining the thickness and the elastic modulus of respectivelayers (i.e., dielectric, adhesive and conductive layers) of the FPCB sothat the neutral plane is located within the thickness or apredetermined range of the thickness of the conductive portion, andmanufacturing the FPCB according to the determined values of thicknessand elastic modulus.

Accordingly, method of manufacturing an FPCB of the present inventioncan be generally divided into two types.

First type is to manufacture an FPCB by suitably determine the thicknessand elastic modulus of a first dielectric portion, a second dielectricportion and a conductive portion (or a cover and a conductive member),and locating a neutral plane within a predetermined range of thethickness of the conductive portion. Second type is to stack a suitablethickness of dielectric layer (or shield layer) on a previously madeFPCB so that a neutral plane is located within a predetermined range ofthe thickness of a conductive portion.

A first type embodiment of the manufacturing method was alreadydescribed with reference to FIG. 5.

That is, the thickness and elastic modulus of respective layers aredetermined based upon above-described Formulae, such as Formulae 1 and 2or Formulae 1 and 3, and an FPCB is manufactured according to thedetermined values.

Here, it is frequent that the material of the respective dielectric orshield layer is previously determined. When the material is previouslydetermined, the elastic modulus is automatically determined as a uniquevalue of the respective material, and it is required to determine onlythe thickness of the respective layer. The geometric center can beeasily calculated when the thickness and elastic modulus of therespective layer is determined. The position Y_(n) of the neutral planeis obtained according to Formula 1 above, and it is designed so that theposition Y_(n) is present in a predetermined range of the thickness ofthe conductive portion.

If not only the elastic modulus of the respective layers but also thethickness of any of the layers are fixed, the thickness of the layer,which is variable, can determined according to above-described Formulae1 and 2 or Formulae 1 and 3 in order to manufacture an FPCB.

Now, a second type embodiment of the method of manufacturing an FPCBwill be described with reference to FIGS. 7 and 8.

As shown in FIG. 7, the FPCB includes a cover 40 and a conductive member50, in which the cover 40 includes first and second layers 41 and 42,and the conductive member 50 includes a conductive portion 30 and athird layer 50.

The first layer 41 has a thickness T₁ and an elastic modulus E₁, thesecond layer 42 has a thickness T₂ and an elastic modulus E₂, theconductive portion 30 has a thickness T_(c) and an elastic modulusE_(c), and the third layer 53 has a thickness T₃ and an elastic modulusE₃.

The position Y_(n) of the neutral plane 35 is not present in the rangebetween D₁ and D₂ of the conductive portion 30. That is, neutral plane35 is spaced from the conductive portion 30 at a predetermined distance,and this represents that the conductive portion 30 is greatly subjectedto tension or compression by bending.

This embodiment is directed to a manufacturing method, which causes theneutral plane 35 to be located in the range between D₁ and D₂ bysuitably adding a dielectric layer or a shield layer to the FPCB, in thestate shown in FIG. 7.

As shown in FIG. 8, the method of manufacturing an FPCB involvessuitably layering a shield on at least one of the top surface and theunderside of the FPCB shown in FIG. 7, so that the neutral plane 35 islocated within a predetermined range of the thickness of the conductiveportion 30.

Referring to FIG. 8, it is illustrated that the position of the neutralplane 35 is adjusted using the shield, that is, a first shield 61layered on the top surface of the cover 40 and a second shield 62layered on the underside surface of the conductive member 50.

Preferably, the thickness and the elastic modulus of the first andsecond shields 61 and 62 can be determined based upon Formula 1 and/orFormula 2 above.

The first shield 61 has a thickness T_(s1) and an elastic modulusE_(s1), the second shield 62 has a thickness T_(s2) and an elasticmodulus E_(s2). These values will be determined in the following.

The thickness T₁ and elastic modulus E₁ of the first layer 41, thethickness T₂ and elastic modulus E₂ of the second layer 42, thethickness T_(c) and elastic modulus E_(c) of the conductive layer 30,and the thickness T₃ and elastic modulus E₃ of the third layer 53 arepreviously determined.

When the values T_(s1), E_(s1), T_(s2) and E_(s2) are inputted,respectively, into Formula 1:

Y _(n)=(Σ(T _(i) *E _(i) *Y _(i)))/(Σ(T _(i) *E _(i)))

Y _(s1) (the distance from a reference plane to the geometric center ofthe first shield 61) can be expressed as in the following Formula:

Y _(s1) =T _(s1)/2+T ₁ +T ₂ +T _(c) +T ₃ +T _(s2)

In addition, Y_(s2) (the distance from the reference plane to thegeometric center of the second shield 61) can be expressed as in thefollowing Formula:

Y _(s2) =T _(s2)/2

As described above, a suitable value is selected based on the relationwith the position Yn of the neutral plane by inputting T_(s1), E_(s1),Y_(s1), T_(s2), E_(s2) and Y_(s2). It is preferable that the value ofY_(n) satisfy Formula 2 or Formula 3 above.

That is, the values T_(s1), E_(s1), T_(s2) and E_(s2) are determined bysuitably selecting from above-mentioned formulas, and based upon thedetermined values, the first shield 61 and the second shield 62 arelayered on the FPCT as shown in FIG. 7, so that the neutral plane 35 canbe located within a predetermined range of the thickness of theconductive portion 30 as shown in FIG. 8.

While the present invention has been shown and described in connectionwith the exemplary embodiments, it will be apparent to those skilled inthe art that modifications and variations can be made without departingfrom the spirit and scope of the present invention as defined by theappended claims.

1. A method of manufacturing a flexible printed circuit board,comprising: adhering a cover, which includes an adhesive layer and atleast one dielectric layer, to a conductive member, which includes aconductive portion and at least one dielectric layer; determining theelastic modulus and the thickness of a shield to be layered on at leastone of the conductive member and the cover, so that a neutral plane islocated within a predetermined range of the thickness of the conductiveportion, the neutral plane forming a substantial interface of tensionand compression in response to bending; and layering the shield on atleast one of the conductive member and the cover, with the elasticmodulus and the thickness of the shield determined so that the neutralplane is located in the conductive portion.
 2. The method of claim 1,wherein the shield comprises: a first shield layered on a top surface ofthe cover; and a second shield layered on an underside of the conductivemember.
 3. The method of claim 1, wherein the elastic modulus and thethickness of the shield are determined according to the followingFormula:Y _(n)=(Σ(T _(i) *E _(i) *Y _(i)))/(Σ(T _(i))E _(i))), andΣT _(x) <Y _(n)<(ΣT _(x) +T _(c)), where Y_(n) is a distance normallyextending from a reference plane to the neutral plane, T_(i) is thethickness of a respective layer of the flexible printed circuit board,E_(i) is an elastic modulus of a material placed in the respective layerof the flexible printed circuit board, Y_(i) is a distance normallyextending from the reference plane to the geometric center of therespective layer, T_(x) is the thickness of the respective layer underthe conductive portion, T_(c) is the thickness of the conductiveportion, and the reference plane is an underside of the flexible printedcircuit board.
 4. The method of claim 3, wherein the determining of thethickness and elastic modulus of the shield comprises locating theneutral plane in a range determined by the following Formula:(ΣT _(x)+0.2T _(c))≦Y _(n)≦(ΣT _(x)+0.8T _(c)).